God is pure Act, with no potentialities. But he does not need the potential, exercised or not, to take on a human nature. What changes in the hypostatic union is not the divine nature, but the human nature that is united to the unchanging divine nature.

God is pure Act, with no potentialities. But he does not need the potential, exercised or not, to take on a human nature. What changes in the hypostatic union is not the divine nature, but the human nature that is united to the unchanging divine nature.

Christ is 100% man and 100% divine. It is not possible for human nature to change the divine, rather the divine can change human nature. God remains completely unchanged even as Christ incarnate in the human form.

Where is my evidence? You’ll have to read the following…

Scripture:

“God is Spirit, and those who worship him must worship in Spirit and truth.” John 4:24

From the Catechism:

237 The Trinity is a mystery of faith in the strict sense, one of the “mysteries that are hidden in God, which can never be known unless they are revealed by God”.58 To be sure, God has left traces of his Trinitarian being in his work of creation and in his Revelation throughout the Old Testament. But his inmost Being as Holy Trinity is a mystery that is inaccessible to reason alone or even to Israel’s faith before the Incarnation of God’s Son and the sending of the Holy Spirit.

253 The Trinity is One. We do not confess three Gods, but one God in three persons, the “consubstantial Trinity”.83 The divine persons do not share the one divinity among themselves but each of them is God whole and entire: "The Father is that which the Son is, the Son that which the Father is, the Father and the Son that which the Holy Spirit is, i.e. by nature one God."84 In the words of the Fourth Lateran Council (1215), "Each of the persons is that supreme reality, viz., the divine substance, essence or nature."85

St. Thomas Aquinas, Summa Theologiae:

Long Top True White Horseshoe Sleeved Religion Secondly, one thing is made up of several things, perfect but changed, as a mixture is made up of its elements; and in this way some have said that the union of Incarnation was brought about by manner of combination. But this cannot be. First, because the Divine Nature is altogether immutable, as has been said (I:9:1; I:9:2), hence neither can it be changed into something else, since it is incorruptible; nor can anything else be changed into it, for it cannot be generated. Secondly, because what is mixed is of the same species with none of the elements; for flesh differs in species from any of its elements. And thus Christ would be of the same nature neither with His Father nor with His Mother. Thirdly, because there can be no mingling of things widely apart; for the species of one of them is absorbed, e.g. if we were to put a drop of water in a flagon of wine. And hence, since the Divine Nature infinitely exceeds the human nature, there could be no mixture, but the Divine Nature alone would remain.

Here I am quoting from Part 3: Question 2: Article 1, but the entire question is worth reading as are Part 3 Questions 1-8. These all deal with various aspects of the Incarnation.

You cannot understand the nature of God by reason alone because it is a mystery. However, St. Thomas Aquinas, being a brilliant man, has applied his ability to reason out the existence of God quite well.

Interesting. What is meant by “united”? How is the human nature “united” to the divine nature: connected in the sense of one nature attached to the other nature, combined in the sense of a chemical compound rather than a mixture, made one nature instead of two, or something else?

Do you also reject all aspects of change brought on by G-d, including the Creation? In this case, G-d may not be changing per se, but through His act and subsequent acts, does there not appear to be a change in the Creator with regard to His interaction with His creation, which did not exist prior to His creating it?

First of all, there is probably nothing more important that can be said about this, than that it is a mystery. The humblest and simplest person can understand the mysteries of God oftentimes better than the learned (not that education should be a hindrance to understanding the mysteries of God, but only if it leads the person to think their reason is sufficient). I think the Incarnation has always struck everyone who learns about it it as extremely paradoxical on many levels, something which cannot be grasped by reason alone. Same with the Cross. Not that it is truly a logical paradox (that is, that it doesn’t make sense), but that there is nothing more difficult to understand than this, and all human attempts to explain it will hit a dead end somewhere or another. Almost everything we learn to be true, we know simply “is” long before we get an explanation as to how and why–so it would be expected that that is true of this mystery more than anything else.

That all being said, here are some things to consider.

First of all, that God became Man specifically, because man is that part of creation made in His image and likeness, corresponding to Him in a way that makes divinity and humanity compatible in one Person. “And God created man to his own image…” (Gen 1:27).

God willed all creation in a single act, from all eternity, but man appeared last in the course of time as creation’s fulfillment. The creation of man was the fulfillment of the first creation, but there is a new creation which has its own process in time and fulfillment. “And I saw a new heaven and a new earth. For the first heaven and the first earth was gone, and the sea is now no more. And I John saw the holy city, the new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband.” (Apoc 21:1-2)Top Religion Long Sleeved True Horseshoe White “If then any be in Christ a new creature, the old things are passed away, behold all things are made new. But all things are of God, who hath reconciled us to himself by Christ; and hath given to us the ministry of reconciliation.” (2 Cor 5:17-18).

Here is an indication that all creation was planned, from all eternity (that is “before” there was time or space, or anything created at all), such that it would be fulfilled by including God in it:Long Religion Horseshoe True Top White Sleeved “But when the fulness of the time was come, God sent his Son, made of a woman, made under the law: That he might redeem them who were under the law: that we might receive the adoption of sons.”

That last part, above, is actually one reason I lean toward the view of Bl. John Duns Scotus (though it is way too soon for me to have a strong opinion on it), that the Incarnation was predestined from all eternity (to be fulfilled in time whether or not man sinned), and that Christ’s role as Redeemer presupposes the fact that He is by nature the one Mediator between God and man. “Since God the Father has chosen us in Christ before the creation of the world, it is simply logical that the first willed, the first loved, the first elected one of the Father is Jesus Christ. . . . What God has determined from all eternity before the creation of the world is fulfilled in time,” says Bl. Gabriel Allegra. http://absoluteprimacyofchrist.org/mediation/

Although I am not certain how to answer your question, I think this should at least help us not assume it necessary that the absolutely active God created a receptive creation, designed such that it would fulfill itself (“when the fulness of time was come”) by receiving Him into it. I was a pantheist for a time, and was fascinated to have my expectation broken, that one way that shows that God really is all-powerful is the fact that He can in fact make something other than Himself, out of nothing. And on top of that, then become man and live in time. Moreover, perfection to Him is so often not perfection to us. “For my thoughts are not your thoughts:nor your ways my ways, saith the Lord.For as the heavens are exalted above the earth,so are my ways exalted above your ways,and my thoughts above your thoughts.” (Is 55:8-9)

One thing we have to remember is that God is outside the material world. And that he created it. God’s nature does not change whether the material world exists or not. He doesn’t need it. He is the same with and without it. This means that he can “blend” with his creation without altering Himself.

As @mrsdizzyd notes about Aquinas, the human and the divine in Jesus Christ are not mixed like one would squish play-dough together and create a new “color.” Christ’s Divine Nature assumed a human form without changing who He was. Just like a human being can wear clothes without changing who he is.

through His act and subsequent acts, does there not appear to be a change in the Creator with regard to His interaction with His creation, which did not exist prior to His creating it?

Yes. That is the question that I have. For example, does God respond to our prayers? The bible says that God changed His mind after hearing a prayer: "Then the LORD relented and did not bring on his people the disaster he had threatened."Exodus 32:12-14 .

One thing we have to remember is that God is outside the material world. And that he created it. God’s nature does not change whether the material world exists or not.

Related to this is the idea that God is, in a manner of speaking, outside of time. However, since he created time, we could just as well say that he is eternally present in every moment of time. We experience time as a progression of moments, and in this timeline we perceive change, but God sees all time as one thing, and he is unchanged by time.

How Jesus experienced time, being fully human and fully divine, is a mystery.